Cystathionine Gamma-synthase from Lemna paucicostata has been partially purified by ammonium sulfate precipitation, gel-filtration upon Sephacryl G-300, and DEAE column chromatography. A 140-fold purification was achieved with excellent recovery of activity. The enzyme was physically separated from threonine synthase. The purified enzyme retains its capacity to utilize HS- in place of cysteine, and can utilize also several O-acylhomoserine derivatives in place of 0-phosphohomoserine. One-dimensional SDS-PAGE studies of the most purified cystathionine Gamma-synthase reveal that 10-15 protein bands are still present. The band representing cystathionine Gamma-synthase has not as yet been unequivocally identified.